tambahkan ini di theme css
contohnya
/home/debian/.themes/Prof-XFCE-2.1-Light-Panel/gtk-3.0/gtk.css
window.thunar grid scrolledwindow.sidebar treeview
{
background: alpha(#f0f0f0, 0.7);
color: #303030;
}
window.thunar
{
background: transparent;
}
.thunar scrolledwindow
{
background: transparent;
background-color: transparent;
}
window.thunar paned scrolledwindow treeview.view:selected:dir(ltr)
{
color: #ffffff;
background: #6b94bf;
}
.thunar toolbar.horizontal button image, .thunar .horizontal.inline-toolbar button image {
-gtk-icon-transform: scale(0.9); }
.thunar toolbar, .thunar .inline-toolbar {
color: #404040;
background-color: #f0f0f0; }
.thunar statusbar {
color: #303030;
margin: 0 -10px;
padding: 0 4px;
background-color: #ffffff;}
kemudian di picom tambahkan in
"100:class_g = 'Thunar'",
rubah angka yang warna merah makin kecil makin transparan